Improvements in fuel-to-cost performance and power output of automobiles have been desired in recent years, and a reduction in the weight of automobile materials has been strongly desired. Also, to strengthen a regulation of pollution, purification of an exhaust gas has also been strongly desired. With such a background, as exhaust system materials for automobiles, SUS430LX and AISI409, as existing ferrite system stainless steels, have been used to reduce the weight of exhaust system materials for automobiles and to accomplish a lower thermal capacity of the components. With the improvement in fuel-to-cost performance and a higher output, the highest temperature of the exhaust gas now reaches about 900.degree. C., and the temperature is about 900.degree. C. in the vicinity of an exhaust manifold and about 600.degree. C. in the vicinity of a front pipe. For these reasons, heat-resistant materials used for these thin sheet structures must satisfy the following material properties:
(1) The materials have a high high-temperature strength and high-temperature strength during use.
(2) The materials have excellent thermal fatigue characteristics and high temperature fatigue characteristics.
(3) The materials have excellent normal temperature workability.
(4) The materials have excellent NaCl-induced hot corrosion resistance.
(5) The materials have excellent oxidation resistance.
(6) The materials form a weld bead shape which does not invite stress concentration.
It is expected that if the high temperature strength is improved and a high temperature strength during use can be secured, a high temperature fatigue as well as thermal fatigue characteristics can be improved. In other words, when the characteristics (1) are satisfied, the characteristics (2) can be accomplished, too. The structure described is welded structure made of thin sheets. Therefore, weldability and thermal fatigue characteristics at the weld portions are important. Accordingly, the item (6) is also very important.
Further, the NaCl-induced hot corrosion resistance (4) has drawn a particular attention in exhaust system components of automobiles in which the thickness of said component has been considerably reduced. When automobiles run on the road on which an antifreezing mixture (consisting primarily of salt) is scattered in the cold season, this antifreezing mixture adheres to exhaust system components of the automobile, particularly to an exhaust manifold and a front pipe, which are heated to about 600.degree. C. by an exhaust gas. Then, the surface of these components is corroded and reduction of the thickness takes place until at last breakage of the components occurs and a critical accident is invited.
Accordingly, an improvement in the high temperature tensile strength and NaCl-induced hot corrosion resistance of the materials of the exhaust system of the automobiles due to the increase in the exhaust gas temperature has become a serious problem.
As technologies for improving the various characteristics described above, the following prior art references are known.
Japanese Unexamined Patent Publication (Kokai) Nos. 64-8254, 3-274245 and 4-74852 describe technology relating to the exhaust manifold. Japanese Unexamined Patent Publication (Kokai) No. 64-8254 discloses a composition in which at least 17% of Cr is added in order to improve the oxidation resistance, which uses Nb as an essential component and Mo as a selective element. However, this reference does not add an element (e.g. Ti.) having higher affinity with C and N than Nb and Mo. Therefore, this reference does not consider how to secure the high temperature strength during use because Nb is in a state where it is likely to form carbonitrides during use. In Japanese Unexamined Patent Publication (Kokai) No. 3-274245, the Cr content is rather high, Nb and Mo are essential and Ti is a selective element. This reference also does not consider how to secure strength during use at a high temperature, and uses Ni and Cu as the essential components.
This composition is different from that of the present invention, inclusive of Cr. In Japanese Unexamined Patent Publication (Kokai) No. 4-74852, the Cr content is lower than that in Japanese Unexamined Patent Publication (Kokai) No. 3-274245, Nb and Ti are the essential elements, Mo and Al are not added, and up to 0.5% of Si is added. This reference does not consider NaCl-induced hot corrosion resistance or oxidation resistance at a lower Cr content.
As to a muffler application, Japanese Unexamined Patent Publication (Kokai) No. 3-264652 can be cited. In the composition of this reference, the Cr content exists in a broad range of 11 to 30%, Ti and Nb are the essential elements, and Mo is a selective element. To examine a NaCl-induced corrosion resistance at a normal temperature, a salt water spray test is carried out at 35.degree. C., and as to NaCl-induced corrosion, the reference describes that at least 18% of Cr and 1.0 to 4.0% of Mo are preferably added. The NaCl-induced corrosion at the normal temperature described in this reference is analogous to a resistance to rust (a resistance to rust growing) in the stainless steel, and it is an ordinary view that this property can be improved by adding Cr and Mo (e.g. N. Suutala et al., "Stainless Steels", '84 Gotebrog, Sweden, p. 240 (1984)). On the other hand, the term "NaCl-induced hot corrosion resistance" means the NaCl-induced corrosion resistance at high temperatures, and represents the phenomenon in which corrosion of a 100.mu. order proceeds in the form of the full surface corrosion having temperature dependence, unlike resistance to rust. Furthermore, as can be seen from FIG. 10 of the accompanying drawings, Cr is hardly effective against NaCl-induced hot corrosion at 700.degree. C. Accordingly, it can be said that the NaCl-induced corrosion at the normal temperature is entirely different phenomenon from the NaCl-induced corrosion at a high temperature. In this way, Japanese Unexamined Patent Publication (Kokai) No. 3-264652 does not at all consider the NaCl-induced hot corrosion property.
As described above, all of the prior art references cited above do not at all teach or suggest how to secure the high temperature strength of the materials of the automobile exhaust system, particularly how to secure the strength during the use at the high temperature, or the improvement in the NaCl-induced hot corrosion resistance.
It is an object of the present invention to provide a heat-resistant ferrite system stainless steel which can simultaneously satisfy the material characteristics, before use, equal or superior to those of the conventional SUS430LX, at a low production cost, and which can particularly simultaneously secure the high high-temperature strength during use and NaCl-induced hot corrosion resistance.
In the material described above, the present invention is also directed to provide a heat-resistant ferrite system stainless steel which can offer the required material characteristics, i.e., oxidation resistance, workability, weld bead shape, etc.